The present invention relates to radiation-curable compositions useful when cured as matrix material for optical fiber ribbons; to optical fiber ribbons containing such matrix material; and to processes for preparing such matrix-containing ribbons.
Optical glass fibers have revolutionized the telecommunications industry. The result has been a tremendous growth in demand for optical fibers which are free of many of the inherent defects of glass fibers.
Immediately after drawing, glass fibers are exceptionally strong and have very few intrinsic defects. However, such pristine fibers are very easily flawed by exposure to environmental conditions including dust and moisture. Therefore, there have been developed in the prior art numerous coatings which are minimally capable of protecting the underlying glass fiber from external harmful forces and which optimally possess properties rendering them capable of obviating one or more of the various potential problems which may deleteriously effect optical fiber performance. Such properties include, inter alia, a glass transition temperature rendering the fiber useful over a large potential temperature use range; a higher refractive index than that of the fiber to refract any errant light signals away from the fiber; rapid cure, e.g., under ultraviolet irradiation; and high impermeability to moisture which may damage the coating or the fiber itself and may cause delamination of the two. Additionally, the adhesion level between the fiber and the coating must be optimized so that the coating will remain adhered to the fiber during use but be easily stripped therefrom, with minimal damage to the integrity of the fiber and the coating, so that the fibers may be easily spliced in the field. Above all, the fiber coatings should display good thermal, oxidative and hydrolytic stability, to protect the underlying fiber over the long term, i.e., over twenty-five years' time.
In certain applications, such as in short haul, fiber-to-the-home uses, a single, coated optical fiber may adequately transmit a signal from one point to the next. However, in most embodiments, a relatively large number of fibers are necessary to transmit a large volume of signals. For example, in the telecommunications industry, aggregates of fibers spanning oceans or continents and containing dozens of individual fibers may be required. Fibers are conveniently aggregated into cables, wherein large numbers of coated optical fibers are laid in parallel and are protected by a common sheathing material such as a layered arrangement which may include fiberglass, steel tape and reinforced rubber cabling material.
When numerous individual coated optical fibers are aggregated into a cable, it is necessary to be able to identify each of the individual fibers. For example, when two cable segments are to be spliced together, it is necessary to splice together ends of each like optical fiber in order for a signal to convey properly. When only a few fibers are contained in a cable, identification may be adequately made by having the coating of each individual fiber be a characteristic color; thus, the splicer may simply match up green fiber to green fiber, red to red, and so forth.
However, when the cable contains one hundred or more fibers, it may become impracticable to use a sufficient number of distinctive inks as to color each fiber distinguishably. Thus, a geometric means of distinguishing each fiber is used. For example, arranging the fibers in a number of layers, each layer containing perhaps twelve ink-coated fibers of different respective colors, will greatly facilitate the task of matching up fibers when splicing.
One practical way by which such spatial ordering of numerous fibers may be accomplished is to create two-dimensional fiber arrays, wherein fibers are situated in a generally planar arrangement, within a given array, with the fibers in the array disposed in parallelism with each other. These arrays are stacked one atop another in a three dimensional structure.
Such arrays are known in the art as ribbons. For example, it is known to prepare a two-dimensional ribbon by forming a "sandwich" of parallel coated optical fibers between two sheets of adhesive-coated Mylar tape, thus affixing the fibers in that configuration. This "sandwich" provides structural integrity and a tack free exterior surface.
However, this arrangement is less than optimal because the tape occupies a substantial proportion of the total volume of the sandwich, so that when several "sandwiches" are stacked to form a cable, an undesirably high proportion of the total cable volume is taken up by tape (rather than by optical fiber).
Thus it has been envisioned to prepare an optical fiber ribbon having a matrix material in which the optical fibers are embedded in the desired generally planar, parallel arrangement. This matrix material should, inter alia, have suitable glass transition temperature; cure rapidly; be non-yellowing; and have high thermal, oxidative and hydrolytic (moisture) stability.
Additionally, the matrix material must be adherent enough to the coated, colored optical fibers to prevent separation of the fibers during processing into cables, but not so adherent as to remove the ink coloration from the individual ink-colored fibers when the matrix material is stripped from the fibers to permit splicing. Removal of the ink from a coated, colored optical fiber is referred to in the industry as "breakout failure"; it makes identification of the individual fibers impossible.
Furthermore, the matrix material must possess solvent resistance, inasmuch as, in the field, splicers typically remove residual matrix and coating material from stripped fibers using a solvent such as trichloroethane or ethanol. Matrix material on an unstripped fiber should not absorb solvent and swell and thus compromise the integrity of ribbon.